Internet Core Protocols: The Definitive Guide

Name

IP Options

Synopsis

Everything an IP system needs to deliver or forward a packet is provided in the default headers. However, sometimes you may need to do something special with a datagram, extending its functionality beyond those services provided by the standard header fields. IP Options provide a way to introduce special-handling services to the datagrams or packets, allowing a system to instruct a router to send the datagram through a predefined network, or to note that the path a datagram took should be recorded, among other things.

Size

Varies as needed. The default is zero bits, while the maximum is 40 bytes (a restriction imposed by the limited space that is available in the Header Length field).

Notes

Options provide special-delivery instructions to devices on the network, and can be used to dictate the route that a datagram must take, or to record the route that was taken, or to provide other network-control services. Options are not mandatory, and most IP datagrams do not have any options defined. However, all network devices should support the use of options. If a device does not recognize a specific option type, then it should ignore the option and go ahead and process the datagram as normal.

By default, no options are defined within the IP header, meaning that this field does not exist. An IP header can have as many options as will fit within the space available (up to 40 bytes), if any are required.

Each option has unique characteristics. For more information on ...